Maynard F. Jordan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maynard F. Jordan Observatory clear sky chart from 2005-11-06 to 2025-03-27.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	18% 8% 3 7% 4 4 4 4 5% 7% 36%	108357
	Night	18% 9% 3 7% 4 4 4 4 5% 6% 35%	110999
Transparency	Day	49% 3 11% 22% 13% 2	111911
	Night	49% 3 11% 22% 13% 2	115610
Seeing	Day	26% 15% 31% 19% 8% 2	16205
	Night	23% 12% 31% 26% 7%	24665

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 12% 4 9% 5% 4 5% 3 4 5% 34%	3575
	11th to 20th	Day	15% 9% 3 10% 5% 4 3 2 4 5% 39%	3570
	21st to end	Day	12% 10% 4 6% 5% 5% 4 4 4 5% 41%	4024
February	1st to 10th	Day	17% 11% 4 7% 4 5% 4 4 6% 7% 33%	1800
	11th to 20th	Day	14% 11% 3 8% 5% 5% 4 3 5% 7% 34%	1847
	21st to end	Day	14% 11% 3 8% 6% 5% 4 4 5% 6% 35%	1650
March	1st to 10th	Day	27% 12% 3 7% 4 4 4 2 4 4 29%	3243
	11th to 20th	Day	19% 13% 3 8% 5% 4 4 4 5% 7% 30%	3590
	21st to end	Day	26% 11% 3 7% 4 3 3 3 5% 6% 29%	3882
April	1st to 10th	Day	22% 11% 4 8% 5% 4 4 3 4 5% 31%	2255
	11th to 20th	Day	19% 9% 2 6% 4 4 4 3 4 6% 38%	2428
	21st to end	Day	24% 10% 2 7% 4 3 3 3 4 5% 35%	2470
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 5% 7% 32%	4106
	11th to 20th	Day	19% 8% 3 8% 5% 6% 6% 5% 5% 7% 29%	4081
	21st to end	Day	16% 7% 2 6% 5% 4 5% 5% 6% 8% 35%	5016
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 4 4 5% 7% 29%	2850
	11th to 20th	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 36%	2850
	21st to end	Day	25% 8% 3 6% 4 4 4 4 6% 7% 28%	2850
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 6% 4 7% 9% 35%	4640
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 6% 9% 37%	4440
	21st to end	Day	17% 7% 3 6% 5% 5% 4 4 5% 7% 37%	4756
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 4 4 6% 7% 33%	4105
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 7% 8% 32%	4045
	21st to end	Day	12% 6% 3 6% 5% 5% 4 3 6% 9% 43%	4532
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 8% 49%	2190
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 4 5% 51%	2090
	21st to end	Day	16% 7% 2 6% 3 3 4 3 5% 8% 42%	2090
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 4 6% 40%	2029
	11th to 20th	Day	19% 7% 2 6% 4 3 4 3 4 6% 40%	1886
	21st to end	Day	22% 10% 3 6% 3 4 4 3 4 6% 35%	1881
November	1st to 10th	Day	19% 6% 3 6% 4 4 5% 4 6% 6% 39%	1761
	11th to 20th	Day	20% 7% 2 6% 4 4 4 4 5% 6% 39%	1735
	21st to end	Day	22% 8% 3 5% 4 3 4 4 4 5% 38%	1600
December	1st to 10th	Day	18% 11% 3 5% 4 3 4 3 5% 7% 37%	2396
	11th to 20th	Day	20% 8% 2 5% 4 3 4 3 4 5% 42%	2618
	21st to end	Day	21% 11% 4 6% 4 4 3 3 5% 5% 34%	3476
January	1st to 10th	Night	13% 12% 5% 10% 5% 3 4 3 5% 6% 35%	6368
	11th to 20th	Night	15% 11% 5% 9% 6% 5% 5% 4 5% 6% 30%	6315
	21st to end	Night	11% 11% 3 8% 4 4 5% 4 5% 7% 37%	6930
February	1st to 10th	Night	15% 11% 4 8% 6% 5% 4 5% 6% 6% 31%	2600
	11th to 20th	Night	12% 11% 4 8% 6% 5% 5% 3 4 5% 37%	2434
	21st to end	Night	15% 12% 3 8% 6% 4 4 4 5% 4 36%	1980
March	1st to 10th	Night	29% 13% 3 6% 3 3 3 3 3 4 30%	3816
	11th to 20th	Night	16% 13% 4 7% 6% 6% 5% 5% 6% 6% 27%	3589
	21st to end	Night	27% 10% 3 7% 4 4 4 4 6% 6% 25%	3889
April	1st to 10th	Night	24% 11% 2 7% 4 3 3 2 4 5% 34%	1923
	11th to 20th	Night	21% 8% 2 6% 5% 5% 3 4 5% 6% 34%	1701
	21st to end	Night	25% 10% 2 6% 4 4 4 3 6% 6% 32%	1710
May	1st to 10th	Night	24% 8% 3 5% 3 3 4 4 6% 8% 33%	2834
	11th to 20th	Night	17% 7% 3 9% 5% 5% 6% 4 6% 8% 27%	2813
	21st to end	Night	13% 7% 2 7% 5% 4 4 3 6% 8% 39%	2577
June	1st to 10th	Night	23% 8% 3 8% 4 4 4 4 7% 6% 29%	1330
	11th to 20th	Night	16% 7% 3 6% 4 3 4 5% 7% 9% 36%	1330
	21st to end	Night	25% 8% 3 7% 4 4 3 3 7% 7% 30%	1330
July	1st to 10th	Night	12% 7% 2 6% 6% 5% 6% 5% 7% 9% 35%	2322
	11th to 20th	Night	15% 7% 4 7% 5% 5% 5% 4 7% 8% 34%	2520
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	2765
August	1st to 10th	Night	19% 8% 3 7% 6% 5% 4 4 6% 6% 31%	2825
	11th to 20th	Night	17% 8% 3 7% 5% 6% 4 5% 7% 8% 32%	2782
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 5% 8% 44%	3124
September	1st to 10th	Night	14% 5% 2 4 3 3 4 4 6% 7% 49%	1985
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 7% 50%	2090
	21st to end	Night	18% 5% 2 5% 4 3 4 3 5% 7% 43%	2090
October	1st to 10th	Night	19% 6% 1 6% 4 4 4 4 5% 6% 41%	2150
	11th to 20th	Night	18% 6% 2 7% 4 4 4 4 5% 7% 40%	2289
	21st to end	Night	24% 10% 2 6% 4 3 3 3 4 6% 35%	2717
November	1st to 10th	Night	18% 7% 3 6% 5% 3 4 3 4 6% 41%	2537
	11th to 20th	Night	18% 7% 3 6% 4 4 4 4 5% 7% 38%	2663
	21st to end	Night	19% 9% 2 6% 4 3 5% 4 5% 6% 36%	2800
December	1st to 10th	Night	18% 9% 3 7% 4 4 5% 4 6% 6% 34%	4778
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 4 5% 40%	5591
	21st to end	Night	22% 11% 3 6% 4 4 4 3 5% 5% 34%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	54% 2 11% 24% 9%	3576
	11th to 20th	Day	50% 2 10% 25% 10% 2	3600
	21st to end	Day	46% 2 10% 18% 19% 5%	4024
February	1st to 10th	Day	51% 3 10% 19% 14% 3	1800
	11th to 20th	Day	51% 3 10% 21% 13% 2	1855
	21st to end	Day	51% 3 10% 21% 11% 4	1650
March	1st to 10th	Day	61% 2 7% 13% 13% 4	3247
	11th to 20th	Day	56% 2 10% 18% 12% 2	3590
	21st to end	Day	58% 2 5% 17% 16% 3	3882
April	1st to 10th	Day	57% 3 7% 16% 14% 4	2255
	11th to 20th	Day	48% 3 9% 24% 15% 2	2428
	21st to end	Day	52% 3 7% 19% 15% 4	2470
May	1st to 10th	Day	53% 3 11% 21% 11% 2	4120
	11th to 20th	Day	55% 2 11% 20% 9% 1	4085
	21st to end	Day	47% 3 13% 21% 15% 1	5016
June	1st to 10th	Day	55% 3 11% 21% 10% 1	2850
	11th to 20th	Day	46% 4 11% 21% 16% 2	2850
	21st to end	Day	55% 4 11% 19% 10% 2	2850
July	1st to 10th	Day	44% 4 14% 25% 11% 1	4568
	11th to 20th	Day	43% 5% 14% 27% 11% 1	4365
	21st to end	Day	47% 4 10% 24% 13% 1	4604
August	1st to 10th	Day	50% 4 14% 26% 6%	4092
	11th to 20th	Day	49% 5% 14% 23% 8%	4084
	21st to end	Day	37% 5% 12% 27% 18% 1	4532
September	1st to 10th	Day	34% 4 13% 29% 18% 1	2190
	11th to 20th	Day	37% 3 8% 29% 21% 3	2090
	21st to end	Day	42% 3 9% 28% 17% 1	2090
October	1st to 10th	Day	54% 4 8% 21% 12%	3367
	11th to 20th	Day	39% 2 10% 33% 16% 1	3114
	21st to end	Day	38% 3 15% 24% 19% 1	3081
November	1st to 10th	Day	46% 3 13% 22% 14% 1	1761
	11th to 20th	Day	47% 4 9% 22% 14% 3	1735
	21st to end	Day	49% 4 14% 24% 8% 1	1600
December	1st to 10th	Day	48% 4 17% 17% 13% 1	2396
	11th to 20th	Day	46% 2 12% 18% 15% 6%	2618
	21st to end	Day	53% 2 11% 19% 12% 3	3476
January	1st to 10th	Night	52% 2 11% 24% 9% 1	6368
	11th to 20th	Night	55% 4 11% 19% 9% 2	6360
	21st to end	Night	47% 3 8% 22% 16% 4	6930
February	1st to 10th	Night	52% 3 10% 20% 13% 2	2600
	11th to 20th	Night	51% 2 9% 21% 14% 2	2437
	21st to end	Night	52% 3 7% 21% 14% 4	1980
March	1st to 10th	Night	61% 1 6% 17% 11% 4	3825
	11th to 20th	Night	57% 2 12% 17% 10% 2	3590
	21st to end	Night	61% 2 4 17% 14% 2	3889
April	1st to 10th	Night	54% 2 7% 17% 17% 3	1923
	11th to 20th	Night	50% 4 7% 23% 14% 2	1701
	21st to end	Night	53% 3 10% 17% 14% 3	1710
May	1st to 10th	Night	50% 2 18% 20% 9% 1	2840
	11th to 20th	Night	55% 3 14% 21% 7% 1	2814
	21st to end	Night	44% 3 18% 22% 12% 1	2578
June	1st to 10th	Night	54% 4 11% 21% 8% 1	1330
	11th to 20th	Night	44% 3 17% 22% 12% 2	1330
	21st to end	Night	54% 4 12% 19% 9% 2	1330
July	1st to 10th	Night	44% 5% 16% 24% 12%	2284
	11th to 20th	Night	47% 4 14% 26% 9%	2472
	21st to end	Night	42% 5% 16% 25% 12% 1	2677
August	1st to 10th	Night	54% 5% 13% 21% 7%	2813
	11th to 20th	Night	48% 6% 16% 20% 9%	2809
	21st to end	Night	37% 7% 13% 24% 17% 1	3124
September	1st to 10th	Night	34% 4 13% 32% 15% 1	1985
	11th to 20th	Night	36% 3 11% 29% 19% 2	2090
	21st to end	Night	41% 3 9% 30% 15% 1	2090
October	1st to 10th	Night	44% 4 12% 26% 13% 1	3592
	11th to 20th	Night	38% 2 10% 31% 18% 1	3800
	21st to end	Night	41% 3 12% 28% 13% 3	4467
November	1st to 10th	Night	46% 3 11% 26% 13% 1	2537
	11th to 20th	Night	46% 3 9% 23% 16% 2	2663
	21st to end	Night	48% 5% 14% 20% 12% 1	2800
December	1st to 10th	Night	50% 4 10% 26% 10%	4778
	11th to 20th	Night	48% 2 10% 20% 18% 2	5592
	21st to end	Night	54% 2 9% 19% 15% 1	7502

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	33% 13% 35% 17% 2 1	200
	11th to 20th	Day	27% 21% 35% 13% 4 2	200
	21st to end	Day	22% 16% 38% 18% 5%	245
February	1st to 10th	Day	31% 16% 27% 23% 3	400
	11th to 20th	Day	28% 24% 37% 10% 2	443
	21st to end	Day	25% 21% 32% 19% 3	414
March	1st to 10th	Day	32% 19% 30% 16% 2	500
	11th to 20th	Day	27% 22% 31% 14% 5% 1	500
	21st to end	Day	32% 21% 25% 13% 5% 3	530
April	1st to 10th	Day	36% 22% 25% 10% 6% 2	460
	11th to 20th	Day	31% 20% 29% 13% 6% 1	452
	21st to end	Day	33% 17% 30% 15% 5% 2	460
May	1st to 10th	Day	31% 16% 33% 15% 4 1	463
	11th to 20th	Day	26% 14% 33% 19% 7% 1	487
	21st to end	Day	26% 16% 32% 19% 7%	648
June	1st to 10th	Day	29% 12% 30% 19% 9% 1	680
	11th to 20th	Day	22% 14% 27% 25% 12% 1	680
	21st to end	Day	29% 10% 29% 23% 9% 1	680
July	1st to 10th	Day	19% 11% 35% 23% 11% 2	605
	11th to 20th	Day	20% 10% 38% 21% 10% 2	484
	21st to end	Day	17% 9% 37% 26% 10% 2	532
August	1st to 10th	Day	17% 9% 34% 28% 10% 2	486
	11th to 20th	Day	19% 9% 30% 28% 10% 4	479
	21st to end	Day	17% 11% 29% 26% 14% 3	539
September	1st to 10th	Day	15% 13% 33% 24% 12% 3	490
	11th to 20th	Day	21% 7% 30% 21% 14% 7%	490
	21st to end	Day	18% 15% 28% 21% 11% 6%	490
October	1st to 10th	Day	22% 15% 25% 20% 15% 4	457
	11th to 20th	Day	24% 17% 27% 21% 9% 2	390
	21st to end	Day	29% 16% 28% 20% 6% 1	429
November	1st to 10th	Day	26% 14% 26% 24% 8% 2	395
	11th to 20th	Day	30% 16% 33% 16% 6%	400
	21st to end	Day	31% 17% 26% 21% 4 1	400
December	1st to 10th	Day	28% 18% 33% 17% 4	277
	11th to 20th	Day	35% 23% 29% 12% 2 1	200
	21st to end	Day	32% 18% 28% 15% 5% 2	220
January	1st to 10th	Night	23% 18% 31% 24% 4	803
	11th to 20th	Night	27% 16% 30% 22% 5%	800
	21st to end	Night	18% 18% 35% 25% 4	880
February	1st to 10th	Night	23% 18% 33% 23% 3	800
	11th to 20th	Night	21% 21% 36% 19% 2 1	783
	21st to end	Night	24% 15% 35% 23% 3	660
March	1st to 10th	Night	28% 15% 32% 23% 2	800
	11th to 20th	Night	24% 17% 34% 21% 5%	800
	21st to end	Night	26% 15% 31% 21% 6%	863
April	1st to 10th	Night	32% 16% 30% 19% 3	760
	11th to 20th	Night	24% 14% 37% 20% 5%	577
	21st to end	Night	31% 12% 31% 21% 5%	570
May	1st to 10th	Night	26% 11% 30% 28% 4	570
	11th to 20th	Night	24% 11% 36% 24% 5%	562
	21st to end	Night	20% 7% 37% 28% 7%	443
June	1st to 10th	Night	25% 7% 32% 29% 7%	380
	11th to 20th	Night	18% 7% 31% 33% 10%	380
	21st to end	Night	28% 4 26% 36% 6%	380
July	1st to 10th	Night	16% 4 35% 36% 9%	370
	11th to 20th	Night	17% 2 26% 43% 11%	368
	21st to end	Night	18% 4 24% 42% 12%	430
August	1st to 10th	Night	18% 5% 30% 37% 10%	568
	11th to 20th	Night	16% 7% 28% 36% 12% 1	562
	21st to end	Night	15% 6% 26% 38% 12% 1	627
September	1st to 10th	Night	16% 6% 30% 33% 13% 1	750
	11th to 20th	Night	18% 8% 27% 29% 15% 3	760
	21st to end	Night	19% 8% 29% 31% 12% 1	760
October	1st to 10th	Night	23% 7% 25% 31% 14% 1	760
	11th to 20th	Night	22% 10% 29% 28% 9% 1	760
	21st to end	Night	30% 14% 28% 23% 5%	836
November	1st to 10th	Night	24% 12% 31% 26% 8% 1	780
	11th to 20th	Night	25% 14% 30% 22% 8% 1	833
	21st to end	Night	27% 16% 28% 24% 5%	900
December	1st to 10th	Night	28% 11% 33% 22% 6%	900
	11th to 20th	Night	26% 18% 31% 21% 4	900
	21st to end	Night	28% 16% 31% 20% 5%	990

Page generated at 2025-03-24 09:04:32. Version ac07

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